1. Field of the Invention
This invention generally relates to an energy efficient transparent film and, more particularly, to a solar film layer(s) capable of transmitting desired wavelengths of light in the visible band, while blocking wavelengths outside the visible band.
2. Description of the Related Art
According to U.S. Dept. of Energy, homes and commercial buildings consume approximately 40% of the energy used in the United States, costing consumers approximately US$200 billion annually [1]. With growing awareness of energy conservation, green building and fuel efficient transportation, the smart windows segment in the energy efficient green technology market has rapidly grown in recent years. Moreover, energy efficient windows are influenced and promoted strongly by governmental regulation (U.S. Dept. of Energy and U.S. Environmental Protection Agency) to meet the criteria of ENERGY STAR label, which accounts for approximately 60% of all the building windows in the United States.
Concurrent with these demands are the different smart windows technologies that currently exist or are emerging in the market that provide important growth opportunities from materials suppliers, glass/window manufacturers, to end users such as builders, and building/home owners. Some of the competing technologies include electrochromic windows, low-e glass windows, and solar control films that can be attached to the existing windows.
Many different types of solar control films have been invented in the past. A conventional solar control film may be comprised of multiple layers of very thin reflective metal such as silver or aluminum, which is deposited on a transparent substrate by vacuum or vapor deposition. However, these films are not a cost effective solution due to the need for large and expensive equipment that increases the manufacturing cost. Furthermore, these films tend to block significant amounts of visible light. Moreover, metal film is highly conductive, so the films interfere with wireless radio and microwave frequency signals that are often transmitted through the building or automobile windows.
Another type of solar film includes a multilayer polymer film, as described in U.S. Pat. No. 7,906,202 [2]. Since these films do not include conventional heat rejecting metal layers, the solar films do not block radio frequency (RF) or microwave signals. However, the reflective and transmissive properties of multilayer polymer film are a function of the refractive indices of the respective layers, and to achieve a significant reflective performance at specific bandwidths many layers are needed, which increases the overall manufacturing cost. These films have varying performance at different viewing angles.
Other types of solar films use inorganic metal oxide particles such as indium tin oxide [3], antimony tin oxide [4], or a mix of different UV and near IR rejecting metal oxide nanoparticles that include iron oxide or hydroxide oxide for UV rejection; and ruthenium oxide, titanium nitride, tantalum nitride, titanium silicide, molybdenum silicide, and lanthanum boride for IR rejection [5]. However, many of these metal oxides particles are either very difficult or expensive to manufacture in the large scale quantities that are needed for the sizable surface area of windows. Furthermore, for many of the metal oxides it is difficult to achieve high transmission across the entire visible wavelength range, resulting an opaque or tint colored film.
Recently, a method to generate aluminum-doped zinc oxide nanocrystals has been disclosed [6]. Although metal-doped zinc oxide nanocrystals give high transmission in the visible wavelength range and reject IR wavelength above ˜1.5 microns (μm), the chemical synthesis requires some fairly expensive reducing agents, which increases the overall material cost of manufacturing.
Finally, many of the common metal oxide particles (including metal-doped zinc oxide) do not reject well in near IR wavelengths below 1.5 μm, a solar spectrum range that includes a significant amount of solar heat energy that is transmitted through the windows.    [1] http://energy.gov/energysaver/articles/challenge-improving-energy-efficiency-buildings-across-nation.    [2] U.S. Pat. No. 7,906,202B2 3M innovative Properties Co.    [3] U.S. Pat. No. 5,518,810 Mitsubishi Materials Co.    [4] U.S. Pat. No. 6,663,950B2 CP Films, Inc.    [5] U.S. Pat. No. 6,060,154 Sumitomo Metal Mining Co., Ltd.    [6] WO2012071507 The Regents of University of California at Berkeley.    [7] Bohren & Huffman, Absorption and Scattering of Light by Small Particles, Wiley-VCH (2004).
It would be advantageous if a solar control film could be manufactured at a very low-cost and still provide good light rejection in both UV and broadband IR spectral ranges.
It would be advantageous if a solar control film could conserve electricity costs not only from cooling, but also from heating by rejecting thermal heat at mid-wavelength IR spectrum (2-8 μm), to provide heat insulation inside a building.